1. Field of the Invention
The present invention relates to a gray voltage generator for a liquid crystal display capable of controlling a viewing angle. More particularly, the present invention relates to a gray voltage generator for a liquid crystal display which can produce a gray voltage that is applied to a liquid crystal panel and control a viewing angle of the liquid crystal.
2. Description of the Prior Art
Conventionally, a driving method which periodically inverts an applied voltage of a liquid crystal panel is used because the liquid crystals become degraded and a noninverted DC voltage has a bad influence on durability of the liquid crystal panel.
Reverse driving methods can be generally classified as a first method by which only a gray voltage is inverted on the basis of the potential of a common electrode and a second method by which the voltage of the common electrode and the gray voltage are both inverted on the basis of predetermined potential. The latter, especially, is called a common electrode reverse method.
The common electrode driving method has an advantage that a compact and cheap driver integrated circuit made by a 5 V complementary metal oxide semiconductor process may be used since reverse width can be reduced to a half by the common electrode reverse method compared with the first method mentioned above.
Regarding the common electrode reverse method, a paper, "8.4-inch Color TFT-LCD with 0.27 mm Pixel Pitch Aims at Industry Standard" by Yoshiharu Kanatani on Pages 68 to 72 of NICKEL ELECTRONICS ASIA/October 1992 proposed the use of a blacklight, a liquid crystal panel, a driver LSI and the common electrode reverse driving method to embody low power consumption and to be operated by a portable 5 V power supply.
The conventional gray voltage generator according to the common electrode reverse method will be explained with reference to the attached drawings.
FIG. 1 is a schematic illustration of a conventional liquid crystal display, and FIG. 2 is a detailed circuit diagram illustrating a conventional gray voltage generator for liquid crystal display.
Referring to FIG. 1, a video signals RGB, horizontal and vertical synchronous signals HSYNC and VSYNC, and a clock signal SCLK are inputted to a microcontroller of the conventional liquid crystal display, and a data signal DATA, various signals CTL1 and CTL2, and a reverse signal RVS are produced. The reverse signal RVS has an inverting period which enables voltage applied to a liquid crystal panel 5 by a source driver 3 to be reversed for each frame.
The reverse signal RVS is inputted to a gray voltage generator 2 and 8 levels of gray voltage are produced, and the 8 gray voltage levels are applied to the source driver 3.
The signal CTL2 from the microcontroller 1 is applied to a gate driver 4, and a gate electrode of each line in the liquid crystal panel 5 is sequentially turned ON by a driving voltage applied in response to the signal CTL2. One of the 8 gray voltage levels in the gray voltage generator 2 corresponding to the data signal inputted by the signal CTL1 is selected in the source driver 3 and is applied to the liquid crystal display 5 for each line.
When the corresponding TFT is turned ON by the gate driver 4, desired information may be displayed in each pixel of the liquid crystal panel 5 by applying the gray voltage applied from the source driver 3 and voltage corresponding to potential difference of a common electrode VCOM, thereby determining optical transmittivity of the liquid crystal corresponding to the applied voltage.
At this point, the 8 gray voltage level signals from the gray voltage generator 2 are reversed per frame according to a reverse period of the reverse signal RVS from the microcontroller, and the voltage applied to the liquid crystal of each pixel in the liquid crystal panel 5 is also reversed per frame.
Referring to FIG. 2, the operation of the gray voltage generator 2 will be explained. The inputted reverse signal RVS is connected to resistor R21 and then inverted and amplified, or amplified respectively, by operational amplifiers OP1 and OP2. One terminal of resistor R22 is connected to the non-inverting input of OP1, while the other terminal of resistor R22 is connected to the ground. A reverse phase signal VB of the inputted signal RVS is produced from the operational amplifier OP1, and an in-phase signal VA of the inputted signal RVS is produced from the operational amplifier OP2. Resistor R23 provides feedback from VB to the inverting input of OP1. Resistor R24 provides feedback from VA to the inverting input of OP2.
The output signals VA and VB from the operational amplifiers OP1 and OP2 are applied to opposite terminals of seven serially connected resistors R1 to R7. The potential difference between the output signals VA and VB from two operational amplifiers OP1 and OP2 is divided by each resistor R1 to R7, and the 8 gray voltage levels formed sequentially are produced from the output terminals of each resistor R1 to R7.
The 8 gray voltage levels are also reversed by inversion of the output signal VA and VB from the operational amplifiers OP1 and OP2 in the output terminals of each resistor R1 to R7 at every reverse period.
However, the conventional gray voltage generator has a disadvantage in that the control of the viewing angle of the liquid crystal, which corresponds to the shift of the gray voltage, can not be made because the output signal of each operational amplifier, sets, the gray voltage without regard to viewing angle differences.
In addition, the conventional gray voltage generator has other disadvantages in that the gray voltage of the gray voltage generator does not take into account the characteristics of the liquid crystal, such as different threshhold voltages of different liquid crystals, and the gray voltage may drop by as much as a kick back voltage when applied to the liquid crystal.